1. Field of the Invention
This invention relates to a reference potential generating circuit for generating a reference potential used for, for example, a sense amplifier in a semimemory device.
2. Description of the Related Art
In general, a power source voltage dependent type or threshold voltage dependent type circuit is widely used as a reference potential generating circuit. The power source voltage dependent type reference potential generating circuit includes a plurality of load elements connected in series between the power source terminal and the ground terminal. The reference potential is derived from a connection node between the load elements. Resistors or depletion type insulated gate field effect transistors are used as the load elements. The reference potential is derived by dividing the voltage between the power source terminal and the ground terminal by using the load elements as voltage dividing resistors. In this case, however, the reference potential derived from the circuit is largely dependent on the power source voltage. Thus, if the reference potential is dependent on the power source voltage and when the power source voltage varies, the reference potential fluctuates. Therefore, if the power source voltage dependent type reference potential generating circuit is used in a sense amplifier circuit, an error operation, for example, erroneous readout of memory data occurs when the power source voltage varies. The power source voltage may be varied by, for example, power source voltage noise.
In contrast, a reference potential from the threshold voltage dependent type reference potential generating circuit is less dependent on the power source voltage. The threshold voltage dependent type reference potential generating circuit functions to generate a reference potential by utilizing a threshold voltage of an insulated gate field effect transistor. That is, the reference potential generating circuit is constituted by connecting a depletion type insulated gate field effect transistor whose gate is grounded between the power source terminal and the series-connected load elements. The reference potential from the reference potential generating circuit is largely dependent on the threshold voltage of the transistor. Thus, the reference potential generated from the threshold voltage dependent type reference potential generating circuit will not greatly fluctuate even if the power source voltage varies, but tends to fluctuate according to variation in the threshold voltage of the transistor. Therefore, if the threshold voltage dependent type reference potential generating circuit is used in the sense amplifier circuit and when the threshold voltage of the transistor is changed, then erroneous operation such as erroneous readout of memory data will occur. The threshold voltage of the transistor may be changed by variation in the transistor characteristics caused in the manufacturing process, for example.
As described above, the output potential of the power source voltage dependent type reference potential generating circuit is little affected by the threshold voltage of the transistor but largely depends on the power source voltage, and the output potential of the threshold voltage dependent type reference potential generating circuit is little affected by the power source voltage but largely depends on the threshold voltage of the transistor. Therefore, the output potential of the conventional reference potential generating circuit will fluctuate according to the power source voltage noise or variation in the transistor characteristics caused in the manufacturing process.
In semiconductor memory devices, for example, if the power sourceb voltage or signals externally supplied are little dependent on the power source voltage and the threshold voltage of the transistor or the like, it is necessary to make the reference potential generated from the reference potential generating circuit little dependent on both the power source voltage and the threshold voltage of the transistor. That is, the conventional reference potential generating circuit does not fully satisfy the requirement for preventing fluctuation of the reference potential when used in the sense amplifier circuit of the semiconductor memory device.